A power CCDF curve is frequently used in RF applications to provide critical information about the signals encountered in RF systems, for example in modern 3rd Generation (3G) digitally modulated radio systems. The power CCDF curve consists of a curve relating the percentage of time a radio signal spends at or above a particular power level. It is usually shown as a graph of the ratio of the instantaneous power to the average power against percentage of time that the signal power is at, or above, the power specified in the X axis. Both axes of the graph are usually logarithmic.
Perhaps the most important application of power CCDF curves is to specify completely the power characteristics of the signals that will be mixed, amplified and decoded in communication systems. For example, 3G systems combine multiple channels resulting in a peak-to-average power ratio that is dependent upon not only the number of channels, but also which specific channels are used. This signal characteristic can lead to higher distortion unless the peak power levels are accounted for in the design of system components, such as amplifiers and mixers.
A typical system can take around 5 seconds to measure and process 100,000 samples. In most known systems, samples tend to be captured at high speed for a short time, then the captured samples are processed and then another batch of samples are taken. Current ADC technologies are capable of running at speeds in the range of 10M samples/second up to, in some cases, several Giga samples per second. What this means is that a typical system will take very short snapshots of a waveform, which can result in missing the elusive high peak levels that have the greatest effect on the resulting measurement numbers.
As well as examining a graph, a user may also request the power level at which a certain percentage of the measurements lie in excess. Typical percentages lie in the range of 0.01% to 0.0001%. For example, if a sample of 100,000 measurements has taken place, if the user requires the power ratio at which 0.01% of samples lie in excess then there will be 10 samples lying above the 0.01% percentage point (0.01%=1/10,000). Normally the ratio of the tenth largest sample to the calculated average power is returned.
It can also be readily seen that 100,000 samples provides only ten samples “above the line” for a 0.01% measurement. Measurements of 0.001% or 0.0001% would require orders of magnitude more time to get the same accuracy. Furthermore, ten samples is, in some circumstances, not a sufficiently large number on which to base an accurate measurement.
A dual port memory is a memory that has two address buses and at least two data buses, one address bus and at least one data bus forming one port. The memory can be accesses at both ports at once provided the addresses are not the same. If the addresses are the same, then if either port is writing to the memory, the result is indeterminate. An updating device, such as an adder, coupled to one port of the dual port memory can update the count value and provide it in the same clock cycle, so that the updated count value is ready to be written on the next clock cycle at either port. Thus, since a particular address in the memory is read in one clock cycle and written in the next clock cycle, it takes half the frequency of the clock cycle to perform an update of a sample into the memory.